Posidoniusor Poseidonius(135?––51? B.C.), a Greek Stoic philosopher. He was the greatest scholar of his time and wrote in many fields, but his works (which have been lost) were apparently given to the uncritical gathering of facts and ideas rather than to original thought. He brought new life to Stoicism, which taught that to be wise one must govern one's life by natural law. Posidonius was born in Syria and eventually settled in Rhodes and became a teacher. He wrote on natural science, mathematics, astronomy, geography, and history. Cicero studied under him.